1. Field of the Invention
The present invention relates to a car-mounted booster for plug-in connection with a mobile telephone set, or portable unit as referred to hereinafter, included in a cellular car telephone system. More particularly, the present invention is concerned with such a booster which reduces the loss of high frequency signals due to the connection with the portable unit.
2. Description of the Prior Art
A conventional car telephone is selectively operable in a portable mode in which a portable unit is removed from a booster, or in a car-mounted mode in which it is plugged in the booster. In the portable mode, the portable unit uses a built-in telephone antenna for transmission of communication signals. In the car-mounted mode, the portable unit is plugged in a car-mounted apparatus made up of a connection unit, which is removably connected, to the portable unit a curled cord connected to the connection unit by a coaxial cable, a power amplifier unit connected to the curled cord and fixed in place in, for example, a luggage compartment, and a car antenna connected to the power amplifier unit.
In the portable mode, the portable unit modulates a carrier signal generated by a first frequency synthesizer by a communication signal and other signals to thereby produce a first transmit signal to be transmitted. The telephone antenna converts the first transmit signal to a radio frequency (RF) signal and then sends it to a base station. The telephone antenna also functions to convert an RF signal sent from the base station to a first received signal, and a receiving amplifier amplifies the first received signal. A first receiving mixer is included in the portable unit and generates a first intermediate frequency (IF) signal in response to the amplified first received signal and a first local oscillation signal generated by a second frequency synthesizer. A demodulator produces a demodulated signal including a communication signal and other signals from the first IF signal.
When the user connects the portable unit to the connection unit and then turns on booster ON means incorporated in the connection unit, the power amplifier unit is activated to set up the car-mounted mode. Then, the user is capable of holding a conversation with a remote base station using the portable unit as a car telephone. As an RF signal sent from the base station comes in through the car antenna, the power amplifier unit effects low noise amplification with a weak second received signal based on the RF signal by a preamplifier and feeds the amplified second received signal to the portable unit. On receiving a first transmit signal from the portable unit, the power amplifier unit amplifies or boosts the signal using the power amplifier and feeds the resulting second transmit signal to the car antenna. In this manner, the power amplifier unit serves to increase the communicable distance between the car telephone and the base station. The power amplifier unit includes two duplexes, i.e., one on the car antenna side for separating the second transmit signal and the second received signal and the other on the portable unit side for separating the first transmit signal and the second received signal undergone low noise amplification.
The RF signal applicable to the car telephone system lies in, for example, the 800 MHz band as prescribed by AMPS (Advanced Mobile Telephone System), and the transmit signal and the received signal are different in frequency from each other. The received signal fed from the power amplifier unit to the portable unit has the same frequency as the RF signal sent from the base station. The frequency of the IF signal is often selected to be about 90 MHz. Since the curled cord is about 5 meters long, it has to be small size, light weight and, in addition, flexible. To meet these requirements, a high frequency signal transmission cable corresponding to a 1.5D-2 W (or RG-174/U as prescribed by MIL standards) coaxial cable is used for the propagation of the second received signal. With such a coaxial cable, however, the transmission loss of the received signal over the curled cord amounts to about 4.3 dB. Since the loss of received signal due to a high frequency signal transmission cable is substantially proportional to the square root of frequency, the transmission loss ascribable to the curled cord will be further aggravated if the RF signal of a car telephone system is shifted to the 1.5 GHz band, such shift which is now under investigation.
The problem with the conventional car telephone is that the total reception noise factor is low since it compensates for the loss of received signals ascribable to the high frequency signal transmission cable of the curled cord and the two duplexers of the power amplifier unit by increasing the amplification gain of the receiving amplifier of the portable unit. Moreover, since the portable unit has no margins as to the voltage to be supplied, power consumption, and volume, the circuit arrangement of the receiving amplifier and that of the receiving mixer are limited. The receiving mixer, for example, causes intermodulation to occur due to the limited circuit arrangement thereof, thereby degrading the reception characteristics.